Conserved bipartite motifs in yeast eIF5 and eIF2B epsilon, GTPase-activating and GDP-GTP exchange factors in translation initiation, mediate bindingto their common substrate eIF2

In the initiation phase of eukaryotic translation, eIF5 stimulates the hydr olysis of GTP bound to eIF2 in the 40S ribosomal pre-initiation complex, an d the resultant GDP on eIF2 is replaced with GTP by the complex nucleotide exchange factor, eIF2B, Bipartite motifs rich in aromatic and acidic residu es are conserved at the C-termini of eIF5 and the catalytic (epsilon) subun it of eIF2B. Here we show that these bipartite motifs are important for the binding of these factors, both in vitro and in vivo, to the beta subunit o f their common substrate eIF2. We also find that three lysine-rich boxes in the N-terminal segment of eIF2 beta mediate the binding of eIF2 to both eI F5 and eIF2B. Thus, eIF5 and eIF2B epsilon employ the same sequence motif t o facilitate interaction with the same segment of their common substrate. I n agreement with this, archaea appear to lack eIF5, eIF2B and the lysine-ri ch binding domain for these factors in their eIF2 beta homolog, The eIF5 bi partite moth is also important for its interaction with the eIF3 complex th rough the NIP1-encoded subunit of eIF3, Thus, the bipartite motif in eIF5 a ppears to be multifunctional, stimulating its recruitment to the 40S preini tiation complex through interaction with eIF3 in addition to binding of its substrate eIF2.